[Amps] Plate coupling capacitor value

[Manfred Mornhinweg]

Bill, Carl,

> There is a better way to tune 10 and 12 meters despite high tube
> output and stray capacitance. A small coil in the anode lead will
> create an "L" network which reduces the plate impedance on those bands
> and permits use of a larger tune capacitor. 

Yes, that's right. And the same technique can very well be extended, so 
that the matching network becomes a cascade of several L networks. The 
nicest thing about this configuration is that the total Q of such a 
matching network can be lower than what's required to implement the full 
impedance transformation in a single L network, let alone a single PI. 
This lower Q provides a larger bandwidth, without having to retune.

You surely know those hybrid ICs used as power amplifier chains in VHF 
and UHF radios. These typically have such cascaded L networks, with 3 or 
4 stages. This allows a 12V 80W module, for example, to cover 136 to 
174MHz with nearly flat performance.

> The actual circuit was to tap the coil

And this technique too is commonly used in tuned solid state amps. The 
coil is implemented as a single trace on the printed board (a 
microstripline), and several chip capacitors are soldered between the 
microstripline and the ground foil, at precisely calculated points. I 
have witnessed non-RF-savvy electronic engineers looking at such 
amplifiers and scratching their heads about how that works...

Manfred

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